1. Field of the Invention
The present invention relates to a density correction technique for use in an image forming apparatus.
2. Description of the Related Art
Electrophotographic image forming apparatuses are required to provide density stability and tone stability of output images. For this reason, U.S. Pat. No. 5,752,126 and U.S. Pat. No. 5,583,644 disclose density correction control that stabilizes the quality of resulting images by forming a test pattern, which is a density correction image, detecting the density of the test pattern, and determining an image forming condition based on the detected density. The density correction control is executed, for example, each time image forming is performed on a predetermined number of sheets.
A variation in the characteristics of an image forming apparatus that affects the density and tone of output images is not always proportional to, for example, the number of image-formed sheets, and accordingly, the following problems may occur when density correction control is performed based on a predetermined number of image-formed sheets. For example, density correction control may be performed at an unnecessary timing despite the fact that the density of output images is stable, or density correction control may not be performed at a necessary timing despite the fact that the density of output images is changing. If density correction control is performed at an unnecessary timing, the productivity of the image forming apparatus decreases. If density correction control is not performed at a necessary timing, the quality of output images decreases.